Power semiconductor devices are widely used to carry high currents and block high voltages. Most power semiconductor devices are fabricated from silicon. However, wide bandgap materials, such as silicon carbide and gallium nitride, offer a number of advantages including higher breakdown field and improved thermal characteristics. As used herein, the term power device means a semiconductor device which is capable of dissipating over 1 watt in normal operation.
Avalanche breakdown is the phenomenon of current multiplication when a semiconductor device is subject to high electric fields. In order to prevent damage to the device it is desirable that the avalanche breakdown occur over a large area, thereby reducing the avalanche current density. Discontinuities at the edges of semiconductor devices create locally large electric fields, tending to produce avalanche breakdown preferentially at the edge instead of uniformly distributed over the entire active area of the device. Edge termination structures are designed and fabricated to remove or smooth the discontinuities at the edge, thereby reducing the otherwise large fields at that location.
In conventional (i.e. Si) semiconductor devices, structures within an active region can be fabricated which will preferentially reduce the breakdown voltage within the active region compared to the edges, forcing breakdown into the active region. Such structures are often formed by the process of diffusion of dopants, which allows a wide range of control over dopant depth and distribution, and thereby control of the avalanche breakdown voltage and location.
Dopant diffusion in wide bandgap semiconductors such as silicon carbide (SiC) is very slow, making it difficult to fabricate deep structures. It is therefore difficult to create regions with different breakdown voltages. As a result, breakdown in wide bandgap semiconductor devices tends to occur at the edges of the devices where electric fields are largest.
Therefore, it can be appreciated that a wide bandgap semiconductor device, with an active region and a termination region outside the active region, in which avalanche breakdown threshold is controlled by the active region rather than by the termination region, is desirable.